I always add all the water needed at one time, into a full gallon...not per quart. This because the water / Glaze combo requires both to properly disperse the Silver Metallic into the medium. Also, you want every coat to posses the same consistency, and doing it in that manner assures such.
Even so, acquiring an eye for how the paint pours out into the Gun's Cup is needed.
Please note that after mixing Water / Glaze / Silver that you strain into a clean container for storage / dispersal. You do not strain each load into each Cupful. All you ever have to do after straining is remix the paint every 2 coats or so to evenly disperse the Silver into the mix...avoiding any possible separation.
I think a lot of your issues revolve around second guessing the viscosity because it's so difficult to judge how much paint is being applied with S-I-L-V-E-R. But you already know what happens when you put too much water into the mix.
At very most, 4 additional oz a Gallon should be considered... and that ONLY because you can feel...or not feel as the case may warrant....that the paint does not seem to be flowing evenly out of the gun.
Also, to judge the height of your pattern, you have to shoot onto a practice sheet....holding the Gun in one place for 2 seconds do at to lay down a saturated footprint. Yeah...the paint will run like crazy pretty fast, but still, the visible wet spot will go much further to allow you to judge flow and delivery than trying to judge by looking at what goes down as you actually spray.